1. Field of the Invention
The present invention relates to a PWM (Pulse Width Modulation) driver for driving a speaker, a ringer or the like for use in a portable device such as a portable telephone driven by a battery.
2. Description of the Prior Arts
In a conventional PWM driver, when a power source has a positive polarity alone (the power source having a single polarity), since a circuit is operated even when no signal is input, a power is wasted. That is, a PWM pulse wave of a duty cycle of 50% or more is output when the input signal has the positive polarity, while the PWM pulse wave of the duty cycle of 50% or less is output when the input signal has a negative polarity, and the PWM pulse wave of the duty cycle of 50% is output when no signal is input.
Furthermore, since an output signal of such a PWM driver is subject to DC offset, a capacitor for cutting off a direct-current component is required to drive a speaker or the like.
Therefore, Japanese Patent Application Laid-open No. 56-156019 proposes a PWM driver wherein the positive-polarity PWM pulse wave is output in response to the positive-polarity input signal, the negative-polarity PWM pulse wave is output in response to the negative-polarity input signal and for setting the PWM pulse wave is set to 0 at the time of no input signal.
On the other hand, Japanese Patent Application Laid-open No. 59-37731 proposes the PWM driver comprising two counters for generating the PWM pulse from PCM data in response to the positive polarity and the negative polarity, wherein each counter outputs the PWM pulse proportional to an absolute value of the input signal, whereby it is not necessary to input a code bit to the counters, so that a clock frequency is reduced in rate.
However, the PWM driver described in Japanese Patent Application Laid-open No. 56-156019 of the above-described conventional PWM driver faces such problem that, since this apparatus needs the power source of both of the positive and negative polarities, it cannot be used for a portable device which generally uses the single-polarity power source and reduces a voltage.
On the other hand, since the PWM driver described in Japanese Patent Application Laid-open No. 59-37731 needs two counters for outputting the PWM pulse, it is difficult to employ this apparatus for the portable device which is advanced in miniaturization and in reduction of the power to be consumed.
Moreover, as described above, since the portable device generally uses the single-polarity power source and reduces the voltage, when the speaker or the like is driven by the single-polarity power source, it is difficult to obtain a high output level.
In addition, for example, in case the PWM pulse wave is obtained from 2's complement PCM data, if an attempt is made to change the output level by a volume or the like, since it is necessary to perform a digital processing by a multiplier, an adder or the like, a problem is caused in which the circuit is complicated.